Cellular networks include radio access nodes, such as base stations or radio access points, which provide radio access services to user terminals that are located in geographical regions—cells—associated with the radio access nodes. Communication between the user terminal and the radio access node is performed over radio links, which are either connections from the radio access node to the user terminal (downlinks) or connections from the user terminal to the radio access node (uplinks).
The cellular networks are often interference limited, i.e. the greatest source of interference on each radio link is transmissions on other radio links, and not e.g. thermal noise. Link quality on one radio link is thus dependent on a relationship between received signal strength on the one radio link and a sum of received signal strengths from all other radio links using the same channel simultaneously. As a consequence, a reduced transmission power on one radio link not only affects the link quality of that radio link negatively but may also improve the quality on other radio links due to reduced interference. Power control of transmissions on the radio links is therefore an important tool when optimising performance in the interference limited cellular network.
The basic idea behind power control is that transmitting at a power level that is higher than needed to achieve an acceptable quality, e.g. perceived speech quality, is only a source of increased interference towards the rest of the cellular network. Therefore, the transmission powers on such radio links should be reduced. In principle, both directions, i.e. uplink as well as downlink, benefit from power control. Power control on the uplink will also have the effect of reducing power consumption and increasing battery time in the user terminals.
The goal for a power control algorithm is to maximize the number of satisfied users in the cellular network. In other words, maximize the number of radio links with a link quality representing acceptable quality or better. Note that the goal is not necessarily to get every single user satisfied. It may, for example, be preferable to let one strong interferer reduce its transmission power below the quality level, if this can reduce interference enough to “lift” more than one other radio link above the quality level.
Details of power control algorithms may differ, but common for all is that information on current radio link quality is needed. Additionally, since the user terminals are normally mobile, radio conditions will change over time, making dynamic updating of quality information necessary. The power control algorithms normally use a number of operational parameters, which establish how the power control algorithm determines the transmission powers of radio links controlled by the power control algorithm. The non-trivial task of selecting appropriate values for the operational parameters lies with the operator of the cellular network.
In e.g. GSM, downlink power control is achieved using measurement reports that are repeatedly transmitted from the user terminal to the radio access node (base station). The reports include information describing the radio link quality measured by a receiver in the user terminal. The information is filtered and compared with a predefined target quality parameter used by the algorithm. In GSM, this target quality parameter is called qdes and defines a link quality level above which the transmitted power can be reduced. A corresponding uplink control does not rely on measurement reports, since it is the cellular network (base station controller) and not the user terminal that controls the power levels also for uplink transmissions. Instead, a command that informs each user terminal of the transmission power to use is sent downlink. The power control in GSM is based on the principles outlined in M. Almgren, H. Andersson and K. Wallstedt, 1994, “Power Control in a Cellular System”, “Vehicular Technology Conference 94”.
In e.g. WCDMA another principle is used. The goal here is to keep each user at a defined speech quality level. This is achieved by an inner loop that regulates towards a link quality target and a slower outer loop, which updates the link quality target dynamically to match the desired speech quality. The outer loop measures other link quality parameters that better correspond to the speech quality but requires longer measurement periods to get reliable values.
Although power control provides an important tool for improving performance, there are still many practical difficulties. Due to physical limitations, transmission powers are as a rule restricted to a dynamic power range, i.e. a predetermined interval extending from a minimum allowed power level (Pmin) to a maximum allowed power level (Pmax). The power control algorithm is hence restricted to choose transmission power levels that lie in the dynamic power range. Moreover, since speech quality is ultimately subjective and not easily measured as such, a corresponding radio link quality level (e.g. measurements of C/I, BER or FER) is normally used instead. The complex relationship between the measured radio link quality level and perceived speech quality makes it difficult to select values for operational parameters of the power control algorithm that relate to link quality entities. Service type (different speech coders), traffic load (the number of simultaneous connections), frequency reuse, frequency hopping etc. may also affect this relationship. Furthermore, it can be expected that the relationship may change over time due to changes in the above-mentioned conditions or other conditions. However, because of the ever-changing conditions, it is not certain that a selection of operational parameters that appeared good at one point time will necessarily be as good at a later point in time. In fact it is quite clear from the above that the power control algorithm cannot be expected to perform in an optimal manner at all times.